There are a number of practical applications in which it is desirable to broadcast low frequency information over a fibre optic network with the minimum possible amount of light tapped from the fibre at each receiving station. This is the case when the fibre is used primarily for a different function, such as in a distributed fibre optic sensor, where the entire length of the fibre is used to measure the distribution of a quantity of interest, e.g. temperature. Where a large number of stations are connected to a single fibre data bus, it is vital that each of these remove as little power from the data bus as possible since this allows a maximum number of stations to be connected. Methods proposed to date include various tapping mechanisms to allow a fraction of the power carried by the fibre to be diverted to an optical receiver assembly, usually consisting of a detector, an amplifier and some data recovery circuitry. Alternatives include active network nodes, where the signal is detected, and passed on down the fibre after regeneration.
All of the methods proposed to date involve diverting power out of the fibre, with or without a break in the fibre. Methods previously proposed for effecting the diversion of power from the fibre include introducing bends in the fibre (and thus inducing some of the power to be diverted from the core), removing part of its guiding cladding (to gain access to the core where the power is normally confined) or heat treating the fibre in order locally to enhance its scattering loss.
These methods are not acceptable in situations where a very considerable number (say greater than 200) of receiving stations are required to be connected to the fibre or where the fibre is also used for distributed sensing, where such power diversion would also affect the power used to probe the fibre for the purpose of sensing, so reducing the signal available in the remainder of the fibre and thus degrading the performance of the sensing system. Moreover in the latter case, diversion of power from the fibre will distort the sensor output by introducing localised losses along the fibre which could be interpreted by the signal processing system as real changes in the parameter to be measured. Moreover, most of the means available to date of diverting the power from the fibre also degrade the mechanical integrity of the fibre and thus may result in the occasional fracture of the fibre in service, a clearly detrimental side effect.
In the present patent application, the concept of using only that light which is inevitably scattered from the fibre through its side to provide one way communication to an array of receiving station sited along the fibre is disclosed. The distinguishing feature of the approach disclosed is that no additional light is required to be tapped from the fibre other than that which would in any case be lost through inevitable propagation mechanisms. In practice, the simple act of gaining access to the fibre (i.e. removing any opaque sheathing materials which the fibre may be protected by in a cable) may induce some excess loss over and above that which would be present in the fibre without such a tap; however it is an essential characteristic of the network disclosed herein that its operation does not require the presence of any excess loss. In other words, the network is designed in such a way that the detectors are directed at the side of an intact fibre and the sensitivity of the receivers is sufficient that they may detect signals using only side scattered light.
This approach to broadcasting information over a fibre optic network has not previously been proposed owing to the very limited data rate it can support, which would, in most communications applications, be totally unsuitable. In the context of the present application, however, the limited data rate is not a serious drawback.